Installing a float valve

I'm going to install one in a stand up reservoir to have water on hand at all times without having to worry about flooding but I was wondering when you install one and it fills up enough to float the float valve what happens to the water that must be building up. I am going to run a RO stealth 100 to the res is why I ask and I am curious if the float valve turns off the water somehow or if I have to turn it off eventually or how it works.

Thanks.

the valve will cut off the supply to the filter thus ending the supply until the float is low enough to restart

here is a pdf that helped explained it to me
as well as gives you an idea on how to set it up

http://www.stealthhydroponics.com/page.php?xPage=support.html

click on external res.

Just bear in mind that if you have a flood table for example, when you flood and the resevoir level drops, it will try fill the main res with water from the reserve res, and may result in it overflowing when the water thats actually on the flood table flows back into it.